Chemical analysis of samples often begins with a sample preparation process to bring an analytical component of interest (the “analyte”) from a solid/semi-solid matrix into aqueous medium or another liquid form. This is because many laboratory instruments used for chemical analysis rely upon analyzing the sample in liquid form. Such laboratory instruments include Inductively Coupled Plasma (ICP), Inductively Couple Plasma Mass Spectrometers (ICPMS), and Atomic Absorption Spectrometers.
The types of samples undergoing sample preparation prior to analysis are diverse and include wastewater, sludge, sediments, soils, rocks, foods, powder, industrial and manufactured products, animal and plant tissue, plastics, oils, steel, greases, coal, cements, and paint chips. The areas of analytical applications are also diverse and include environmental, geological, food, agriculture, forestry, pharmaceutical, and industrial quality control. One common trait among these applications is that, in most cases, the sample undergoes sample preparation before analyzing the sample. There are different types of sample preparation procedures for dissolving the analyte into liquid form such as digestion or another type of dissolution. The following are a few examples of these sample preparation procedures.
Acid digestion is a procedure in which a sample reacts with an acid to dissolve the sample partially or completely into liquid form. Generally, acid digestion is carried out in a beaker placed on a hot plate. This procedure uses large volumes of volatile acids, which can evaporate and escape into the environment, and thus represents an environmental concern. Accordingly, acid vapors are often vented into large expensive fume hoods with exhaust scrubbers. Unfortunately, the scrubbers produce large volumes of acidified wastewater, which still represents an environmental disposal issue. Acid digestion also has a number of other problems. In particular, acid digestion can take many hours, involves continuous monitoring, and tends to be manual and labor intensive. Acid digestion is also prone to loss of the analyte through vaporization, contamination problems, and generally has poor precision. It is also difficult to automate and computerize the acid digestion process. The handling of hot acid also represents a safety concern.
In some laboratories, acid digestions are performed using “hot block” digestion vessels, which are large heated blocks having a number of openings for receiving test tubes containing samples and acid. While this allows some degree of automation and control, acid digestion in a hot block is still prone to the other disadvantages noted above.
Microwave acid digestion is another sample preparation process whereby a sample and acid are placed into a closed vessel and heated by microwave radiation. Volatile elements are contained within the closed vessel, which can offer better control of exhaust fumes and can reduce environmental impact. Microwave acid digestion also tends to use less acid compared to hot block digestion because the acid is contained within the closed vessel. However, microwave acid digestion still suffers from a number of problems. For example, some samples can take longer to digest in comparison to acid digestion in a beaker or hot block. Furthermore, the pressurized closed vessels can be expensive to make, hard to clean, and difficult to work with. Sample sizes are often limited to 0.2-1.0 grams. Another drawback is that the digestion vessel is often made from TEFLON™, which limits the maximum digestion temperature to about 245° C., otherwise the TEFLON™ lining might distort or deteriorate and can contaminate the sample. With these limitations, microwave digestion can be hard to automate, expensive, and typically results in low production rates with limited batch capacity. Accordingly, while microwave acid digestion might be appropriate for low volume laboratories that focus on digesting certain difficult samples, the process is less attractive to high volume laboratories, which tend to focus on productivity and costs while analyzing a diverse range of samples.
Apparatus, systems and methods for preparing samples for chemical analysis are described in PCT Patent Application No. WO2011/054086, which was filed in the name of the present inventors. The system comprises at least one sample container, and a container receptacle apparatus for receiving the sample container. The sample container comprises an elongate tubular body having a crucible portion proximal to a closed end for receiving a sample therein, and an expansion portion proximal to an open end. The container receptacle apparatus comprises a housing having a heating compartment, a cooling compartment spaced apart from the heating compartment, and an insulating region located between the heating compartment and the cooling compartment. The heating compartment is shaped to receive the crucible portion of the sample container, and the cooling compartment is shaped to receive the expansion portion of the sample container. The apparatus also includes a heating mechanism for heating the sample within the crucible portion of the sample container, and a cooling mechanism for cooling the expansion portion of the sample container.
While the apparatus, systems and methods described in WO2011/054086 were capable of overcoming one or more of the problems identified above in respect of conventional sample preparation procedures, the inventors have made further refinements and improvements as now described herein.